Motor driver circuits are essentially small current amplifiers or transconductance amplifiers. In a small current amplifier, the input is a control current and the output is a current that can be used to drive a motor. In a transconductance amplifier, the input is a control voltage and the output is a current that can be used to drive a motor. As can be seen, the function of these circuits is to take a low-current or voltage control signal, and turn it into a proportionally higher-current signal. For example, in a motor driver circuit, the control signal may be on the order of microamps or milliamps, and the motor may require amps for operation.
Typical in such motor driver circuits is an ancillary current sensing circuit. Current sensing is required for accurate control of the device under control. Current limiting in these circuits is essential. For example, if a motor is stalled, it might accept large currents that can destroy the FETs in the motor driver circuit, such as an H-bridge. One method of sensing current that is suitable for current control or limiting is to measure the current that is flowing through the motor. If the current is above a certain threshold, then the motor driver control circuit operates to turn the FETs in the H-bridge off.
Some motor driver circuits utilize pulse width modulation (“PWM”) techniques to more energy efficiently control the controlled device. However, these circuits that have an on-board micro-controller generating a PWM signal to drive a motor have certain drawbacks. For example, the micro-controller can be configured send a signal to reduce the PWM ratio if an over-current status is detected. However, there is a latency between detection of the over-current condition and the response. Disadvantageously, this delay can result in an over-current condition before the PWM ratio is reduced, thus damaging sensitive devices.
Conventional current sensing arrangements often use a sense resistor, across which the voltage is amplified in a first amplifier and then sent to a second amplifier circuit. The output of the first amplifier can be low pass filtered to filter out motor noise. The second amplifier compares the amplified sense resistor voltage to a reference signal and if the amplified sense resistor voltage is higher than the reference signal, then the PWM signals driving the H-bridge FET drivers can be diverted to ground or otherwise modulated to reduce current to the motor. These arrangements consume power and require a large area to implement the design.
Some circuits sample the current through a main power FET using a much narrower FET placed in parallel with the main power FET. In such case, the measured, or sensed, current depends on the ratio of the widths of the two FETs, assuming they have the same terminal voltages. In these arrangements, the measured current is dependent on the current flowing through the narrower FET as well as the temperature, and thus accurate control is difficult to achieve.
What is desired is an improved current sensing circuit and method having improved accuracy of current sensing with at least one sense FET, that is adapted to provide programmability, filtering, noise reduction, and improved precision in an area efficient scheme.